Composition, structure, bonding and thermoelectric properties of "cuT₂P₃" and "cuT₄P ₃", members of the T_1-x(CuP₃)_x series with T being Si and Ge

Through electron microprobe analysis, X-ray and neutron diffraction, it has been established that "CuT₂P₃" and "CuT₄P₃" (T = Si, Ge) adopt the cubic or tetragonally distorted zinc blende structures in which two element mixtures are present on both atomic sites. One site contains the Cu/T mixture while the other site is occupied by T and P. The structure of "CuT₂P ₃" and "CuT₄P₃" can be derived from that of silicon or germanium, in which the single Si or Ge site is broken into two independent sites by the preferential Cu and P substitution. The phases appear to be members of the extended series with a general formula of T _1-x(CuP₃)_x. The Cu-P ratio of 1: 3 provides 4 e^- per atom and optimizes the atomic interactions. Thermoelectric performance of "CuSi₂P₃", "CuGe ₂P₃" and "CuGe₄P₃" was evaluated from low temperatures to 400 K through resistivity, Seebeck coefficient and thermal conductivity measurements. The Ge-containing phases show a metallic-type behaviour and "CuSi₂P₃" is semiconducting with a narrow band gap. The ZT values are bigger for the Ge-containing phases and reach values of 8.49 × 10^-3 for "CuGe₂P₃" and 1.09 × 10^-2 for "CuGe₄P₃" at room temperature.